The properties usually quoted for silicon nitride and other silicon nitride based materials such as Sialon make them ideal candidates for certain engine components. These properties usually consist of high strength, wear resistance, high decomposition temperature, oxidation resistance, excellent thermal shock properties, and resistance to corrosive environments. One major difficulty, however, is in fabricating suitable shapes with these desirable properties.
Although it has recently been shown that, with suitable additives, silicon nitride can be sintered to reasonably high density, the technique has not yet been developed to the point of wide application, and the mechanical properties of the sintered products deteriorate at high temperature, probably as a result of the additives employed. The two methods usually used to fabricate silicon nitride based ceramic bodies are called "reaction sintering" and "hot pressing."
In reaction-sintering, the required shape is first made from compacted silicon powder which is then nitrided in molecular nitrogen or a suitable nitrogen-containing atmosphere at about 1400.degree. C. to give a product averaging about 25% in porosity, although current best practice can produce materials with about 15% porosity. The reaction sintering process usually consumes several days and employs a complex heating cycle. To speed up nitriding, at least 1% Fe is usually added. The original dimensions of the silicon compact remain virtually unchanged during nitriding and therefore quite complex shapes can be obtained, such as turbine blading.
The alternative is to hot press silicon nitride powder with suitable additives under pressures of several thousand psi in a graphite die at about 1700.degree.-1800.degree. C. to produce a silicon nitride product which is essentially fully dense. The highest strength material, at least at room temperature, is obtained only by hot pressing, but this is expensive and is limited to fairly simple shapes. At high temperatures there is some deterioration of mechanical properties, attributed to the hot pressing additives.
A reaction sintered material is fabricated easily, simply by the nitriding process. Unfortunately, it is not strong enough for many applications and, owing to its porosity and consequent high surface area, not very oxidation resistant. However, its mechanical properties do not deteriorate at high temperature.
Accordingly, what is needed is a convenient method that will selectively permit the bonding of either hot-pressed or reaction sintered ceramic bodies into a shape not achievable in one body except by expensive machining. In some instances there is a need to combine the best qualities of both hot-pressed and reaction sintered silicon nitride bodies, permitting one to be used for certain complex components and the other to be used for high strength components, the components then being bonded together in an assembly.
In other instances it may be desired to combine two or more hot pressed or reaction sintered bodies; some shapes are not achievable even by reaction sintering because of the limitations on powder compaction techniques (cold pressing, injection molding, slip casting, etc.), such as when interior spaces with no exterior access are to be formed.
The prior art has approached bonding of ceramic bodies principally by two methods, (a) a chemical bond which results from the use of high pressure over extremely long periods of time, and (b) a predominantly mechanical bond. The type of chemical bonding used here-to-date can be exemplified by reference to a typical mode for making a ceramic gas turbine blading assembly. The blade ring for the assembly is formed first by the method or reaction sintering. The blades accordingly have some degree of porosity but of course are formed in complex shapes and are silicon nitride containing a small amount of iron (about 1%), and trace impurities, as well as an oxide layer. The blades are then put into a die; silicon powder is packed between the blades and reaction sintered to form webs; next, more powder is packed about the blades and webs and reaction sintered to form a strong casing. Silicon nitride powder is then placed in the center of the blade, webs and casing assembly and hot-pressed to form a hub of the silicon nitride. The reaction sintered assembly acts as a die and must withstand considerable pressures. Pressing is carried out under a temperature of about 1700.degree. C. for several hours of continuous hot-pressing. In the process, of course, several of the blades may be destroyed by the pressure applied. A bond is created between the hot-pressed hub and the reaction sintered blading possibly by virtue of a slight diffusion of the pressing aides, such as magnesium oxide or yttrium oxide into the reaction sintered material. The long period at high temperature typically causes deterioration of the blades. The total process including removal of the webs and casing takes several weeks.
Mechanical bonding has been carried out by the prior art in a variety of different ways. For example, an aqueous suspension of silicon containing a binder (e.g. ammonium alginate) is permitted to penetrate into the pores of the silicon nitride body; the binder is driven off by heat and the silicon is nitrided to form a silicon nitride bridge. This is considered a predominately mechanical bond and is best illustrated in U.S. Pat. No. 3,966,885. Unfortunately, this method will not work well with hot-pressed silicon nitride because of the inability of the silicon to penetrate this dense material.
What is needed is a method by which interfacing surfaces of silicon nitride based bodies, whether sintered, hot-pressed or reaction sintered, can be joined together without the necessity for high pressure and without the need for long periods of heating. it is hoped that a portion of the interfacing bodies of silicon nitride can be converted to a new ceramic system by short time heating without the use of pressure.